One of tasks in data transmission is to improve a throughput by minimizing the rate of occurrence of transmission errors. For example, if data of 100,000 bits is transmitted in a transmission path with a bit error rate of, for example, 0.01%, an error of 10 bits occurs. On the other hand, when data of 1,000 bits is transmitted, an error of 0.1 bit occurs. In other words, the rate of occurrence of transmission error decreases with the decrease in the data length.
A retransmission control method that reduces the possibility of occurrence of transmission errors is available (e.g., Patent document 1 explained below). In the retransmission control method, the rate of occurrence of the transmission errors is suppressed in next data transmission by retransmitting data with a shorter data length than a data length when a transmission error has occurred.
To implement the retransmission control method, the reception side must send a control signal as delivery confirmation back to the transmission side. More specifically, if no transmission error occurs, then the reception side sends ACK (ACKnowledgment) back to the transmission side, while if a transmission error has occurred, then the reception side sends NACK (Negative-ACKnowledgment) back to the transmission side.
The method of controlling retransmission upon occurrence of transmission error is explained here. At first, as the most basic method, there is a Stop & Wait method which is a method of deciding data to be transmitted next after ACK/NACK is received. If the Stop & Wait method is used, there is a Go back to N method as a method of solving a problem that a time up to confirmation of delivery becomes an idle time. The Go back to N method is such that transmission efficiency is enhanced by preparing a transmission buffer in the transmission side and performing data transmission even during the time until the delivery is confirmed. There is also a Selective Repeat method such that the transmission efficiency is further enhanced by preparing a reception buffer in the reception side. Moreover, a transmission method that combines the retransmission control with the error correction method is widely used in wireless communication.    Patent document 1: Japanese Patent Application Laid-Open No. H03-099538